Suspension polymerization of either vinyl chloride monomer, or a mixture of vinyl chloride monomer and another copolymerizable monomer, is generally conducted in the following manner. A polymerization vessel fitted with a reflux condenser, a jacket, and a cooling baffle or coil or the like is charged with an aqueous medium, a dispersant (a suspension agent), and a polymerization initiator, as well as various other additives, as required. Subsequently, hot water is passed through the jacket to increase the temperature inside the polymerization vessel to a predetermined polymerization reaction temperature, thereby initiating polymerization. By subsequently passing cold water through the jacket, baffle, coil, and reflux condenser to remove the heat generated by the polymerization reaction, the polymerization reaction is allowed to proceed with the temperature inside the polymerization vessel maintained at a predetermined reaction temperature.
In recent years, attempts have been made to improve productivity by shortening the polymerization time. One method of shortening the polymerization time involves increasing the quantity of polymerization initiator added, thereby increasing the polymerization reaction rate. However, the ability to remove heat from the polymerization vessel is limited, and the quantity of polymerization initiator can only be increased within the bounds allowed by this heat removal capability. As a result, there are limits to how much the polymerization time can be shortened using this method.
One example of a proposed process that enables better shortening of the polymerization time is a process in which, by combining a high-activity polymerization initiator and a low-activity polymerization initiator, the reaction rate can be controlled at both the initial stages of polymerization and the closing stages of polymerization, thereby reducing the polymerization time (patent reference 1). According to this process, the heat removal capability of the polymerization vessel can be utilized effectively in the initial stages and closing stages of the polymerization, but in the middle stages of the polymerization, the heat removal capability is excessive, and can not be effectively utilized. This tendency is particularly marked in those cases where a reflux condenser is used as one of the heat removal devices.
Another proposed process that enables further shortening of the polymerization time is a process in which a high-activity, oil-soluble polymerization initiator is added in the period following commencement of heat removal using the reflux condenser, through until a certain polymerization conversion rate is reached (patent reference 2). According to this process, the heat removal capability of the polymerization vessel can be used effectively, and the vinyl chloride-based polymer can be produced with good productivity. However in this process, the high-activity, oil-soluble polymerization initiator added during the polymerization requires some time to disperse uniformly through the reaction mixture inside the polymerization vessel, meaning the concentration of the polymerization initiator develops an uneven distribution, which causes an increase in the number of fish eyes in the product polymer. Furthermore, because the high-activity, oil-soluble polymerization initiator is added to the polymerization vessel, containing the unreacted monomer, over a long period during the polymerization, polymer scale tends to adhere to the inside of, and the region around the outlet of, the supply pipe for the high-activity, oil-soluble polymerization initiator. As a result, the pipe can sometimes become blocked. Furthermore, because the adhered scale can break away and become incorporated within the polymer, the level of fish eyes within the product tends to increase.
Patent Reference 1: Japanese Laid-open publication (kokai) No. Hei 6-166704
Patent Reference 2: Japanese Laid-open publication (kokai) No. Hei 7-82304